查看更多>>摘要：Wearable devices,interactive human-machine interface equipment,wireless sensors,and small-scale cleaning devices play crucial roles in biomedical implantation,disease treatment,health monitoring,environmental purification,etc.These devices require a sustainable energy source to work effectively.With the consideration of the global energy crisis and environmental pollution,researchers are exploring new,stable,and environmentally friendly methods to power these low-powered devices.Mechanical energy is one of the most abundant natural energy sources.Converting mechanical energy from the ambient environment or host structures into electrical energy via the direct piezoelectric effect is an efficient energy harvesting technique.This paper reviews the application of advanced piezoelectric materials,and small-scale self-powered and self-sensing piezo-electric devices at the cubic centimeter scale in energy harvesting and health monitoring of human,animal,machinery,roads,bridges,as well as the pollutant degradation of the environment.Some of these devices have the capability to not only harvest mechanical energy but also enable real-time monitoring and analysis of the electrical signals generated by the direct piezoelectric effect,facilitating prompt decision-making and appropriate responses.In addition,potential challenges and future prospects of small-scale self-powered and self-sensing piezoelectric devices are discussed.
查看更多>>摘要：The efficiency of the actual Stirling engine is much lower than the ideal Carnot cycle efficiency.To obtain more precise efficiency for Stirling engines,this paper proposes a modified Stirling cycle with a more accurate heat transfer process in Stirling engines based on a thermodynamic function called available potential.The finite-time thermodynamic method is used to analyze the model performance under constant heat source temperature,finite temperature difference heat transfer,and incomplete regenerative processes.A new polytropic process is introduced to model the heat transfer between the working fluid and external heat sources in which only heat above ambient temperature is converted into technical work.The regenerator is divided into numerous smaller heat reservoirs with individual temperature to analyze the incomplete regenerative processes.The expressions of the output power and thermal efficiency are obtained based on the modified irreversible Stirling cycle,and the effects of irreversible losses are analyzed to evaluate the performance of the proposed model.Results indicate that the efficiency of the modified cycle is much lower than that of the ideal Stirling cycle with an isothermal process.With the increase of the average heat transfer temperature difference,there exists an optimum value for the power of the irreversible cycle.The optimum power of the model was obtained for varying thermodynamic parameters by optimizing the average heat transfer temperature difference between the hot and cold sides.To optimize the irreversible model,the multi-objective optimization analysis is carried out based on NSGA-Ⅱ,which results in an optimized output power of 40.87 kW and an optimized thermal efficiency of 44％.
查看更多>>摘要：Effective thermal conductivity and thermal tortuosity are crucial parameters for evaluating the effectiveness of heat conduction within porous media.The direct pore-scale numerical simulation method is applied to investigate the heat conduction processes inside porous structures with different morphologies.The thermal conduction performances of idealized porous structures are directly compared with real foams across a wide range of porosity.Real foam structures are reconstructed using X-ray computed tomography and image processing techniques,while Kelvin and Weaire-Phelan structures are generated through periodic unit cell reconstruction.The detailed temperature fields inside the porous structures are determined by solving the heat conduction equation at the pore scale.The results present that the equivalent thermal conductivity of Kelvin and Weaire-Phelan structures is similar to and greater than that of the real foam structure with the same strut porosity.The thermal tortuosity of real foam structure is relatively larger and the heat conduction path becomes straighter by adopting the anisotropic design.The thermal tortuosity of the fluid channels for Kelvin,Weaire-Phelan,and real foam structures is close to one.The thermal conductivity of porous structures with heat transfer fluid increases as the thermal conductivity ratio of fluid to solid becomes larger.A small porosity of porous media leads to a larger equivalent thermal conductivity due to the dominant contribution of porous skeleton in the heat conduction process.Correlations derived from parallel and series models,as well as the Maxwell-Eucken models,provide decent predictions of effective thermal conductivity,with an average error of less than 8％in the entire range of thermal conductivity ratio.
查看更多>>摘要：The enhancement of pool boiling heat transfer using porous media has been extensively studied.Although the two-phase distribution and evolution in porous media are crucial to the heat transfer performance,including the critical heat flux(CHF)and heat transfer coefficient(HTC),direct observation of the two-phase flow inside the media is limited owing to the blockage of the direct view from the porous structures.In this study,pool boiling visualization experiments were conducted on porous samples with different throat widths in deionized water.The results showed that the HTC increased with the throat width.Additionally,the growth-contraction cycle of the vapor region and the formation and drying of the wall liquid film inside the porous media were investigated.The vapor region,including the maximum and minimum areas in the boiling cycle,was quantitatively described.Furthermore,the relationship between the minimum gas-phase area and HTC peak was identified.A one-dimensional transient model was developed considering solid skeleton heat conduction,liquid film evaporation,and vapor region growth to quantitatively study the influence of heat flux on the internal two-phase flow.The model successfully captured the maximum gas-phase area,duration of boiling cycles,and HTC trends at specific heat fluxes.The results of this quantitative study provide insights into the internal two-phase distribution and evolution induced by pool boiling.
查看更多>>摘要：Membrane fouling inevitably occurs during nanofluidic reverse electrodialysis.Herein,the impact of multi-fouling on the energy conversion performance of negatively charged conical nanochannels under asymmetrical configurations is systematically in-vestigated.The results reveal that in Configuration I,where a high-concentration solution is applied at the tip side,at small concentration ratios,multiple foulings reduce the electric power.In Configuration Ⅱ,where a low-concentration solution is applied at the tip side,multiple foulings near the base side contribute to the electric power.Any fouling that formed near the low-concentration entrance diminished the electric power and energy conversion efficiency.Multi-fouling lowered the electrical power consumption by 69.27％and 99.94％in Configurations I and Ⅱ,respectively.In Configuration I,the electric power first increased with increasing fouling surface charge density,reached its maximum value,and thereafter decreased.In ConfigurationⅡ,the electric power first decreased with increasing fouling surface charge density,reached its minimum value,and thereafter increased.Large negative or positive charge densities of fouling contribute to the electric power and energy conversion efficiency.
查看更多>>摘要：Gesture recognition has diverse application prospects in the field of human-computer interaction.Recently,gesture recognition devices based on strain sensors have achieved remarkable results,among which liquid metal materials have considerable advantages due to their high tensile strength and conductivity.To improve the detection sensitivity of liquid metal strain sensors,a sawtooth-enhanced bending sensor is proposed in this study.Compared with the results from previous studies,the bending sensor shows enhanced resistance variation.In addition,combined with machine learning algorithms,a gesture recognition glove based on the sawtooth-enhanced bending sensor is also fabricated in this study,and various gestures are accurately identified.In the fields of human-computer interaction,wearable sensing,and medical health,the sawtooth-enhanced bending sensor shows great potential and can have wide application prospects.
查看更多>>摘要：A versatile sensing platform employing inorganic MoS2 nanoflowers and organic poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate)(PEDOT:PSS)has been investigated to develop the resistive and capacitive force-sensitive devices.The microstructure of the sensing layer heightens the sensitivity and response time of the dual-mode pressure sensors by augmenting electron pathways and inner stress in response to mechanical stimuli.Consequently,the capacitive and resistive sensors exhibit sensitivities of 0.37 and 0.12 kPa-1,respectively,while demonstrating a remarkable response time of approximately 100 ms.Furthermore,it is noteworthy that the PEDOT:PSS layer exhibits excellent adhesion to polydimethylsiloxane(PDMS)sub-strates,which contributes to the development of highly robust force-sensitive sensors capable of enduring more than 10000 loading/unloading cycles.The combination of MoS2/PEDOT:PSS layers in these dual-mode sensors has shown promising results in detecting human joint movements and subtle physiological signals.Notably,the sensors have achieved a remarkable precision rate of 98％in identifying target objects.These outcomes underscore the significant potential of these sensors for integration into applications such as electronic skin and human-machine interaction.
查看更多>>摘要：In this study,we comprehensively investigated the scaling law for elastic properties of three-dimensional honeycomb-like graphenes(3D graphenes)using hybrid neural network potential-based molecular dynamics simulations and theoretical analy-ses.The elastic constants were obtained as functions of honeycomb hole size,denoted by the graphene wall length L.All five independent elastic constants in the large-L limit are proportional to L-1.The associated coefficients are combinations of elastic constants of two-dimensional graphene.High-order terms including L-2 and L-3 emerge for finite L values.They have three origins,the distorted areas close to the joint lines of 3D graphenes,the variation in solid angles between graphene plates,and the bending distortion of graphene plates.Significantly,the chirality becomes essential with decreasing L because the joint line struc-tures are different between the armchair and zigzag-type 3D graphenes.Our findings provide insights into the elastic properties of graphene-based superstructures and can be used for further studies on graphene-based materials.
查看更多>>摘要：Micro-/nano-indentation has become prevalent in evaluating the mechanical characteristics of biological samples,such as cells and tissues.However,the existing contact models describing conical indentation ignore the joint effects of surface energy and substrate,and consequently cannot accurately extract the Young's modulus of biological samples deposited on substrate.Through finite element methods,we examine the conical indentation of biological films on substrates while taking surface energy into account.Based on the dimensional analysis,the explicit relationship between load and indentation depth is achieved for films with their moduli varying from 0.001 to 100 times that of the substrate.If the classical Sneddon's model was employed to analyze the load-depth data,the measured modulus could reach 18 times the real modulus for films on harder substrates,but only 4％of the real modulus for films on softer substrates.Meanwhile,in micro-/nano-indentations,neglecting the contribution of surface energy would result in an overestimation of the Young's modulus of films depending on the contact size.The analytical expression provided here can be utilized to precisely deduce the mechanical characteristics of biological films deposited on substrate from the load and indentation depth data of a conical indentation.
查看更多>>摘要：We propose a large combined moving component composed of carbon fiber reinforced polymer(CFRP)laminates for making lightweight machine tools with high dynamic performance.The accurate dynamic prediction of composite machine tools is essential for the new generation machine tool.This paper aims to address two challenges in numerical dynamic modeling and the design of composite machine tools to enhance development efficiency.(1)Anisotropic composite laminates,which form the composite machine tool,exhibit coupling in various directions.We propose the generalized continuity condition of the boundary to tackle this dynamic modeling challenge.(2)Composite machine tools feature numerous composite-metal coupled structures.The mechanical model correction of isotropic metals is performed to address their dynamics.We take the example of a five-axis gantry machine tool with composite moving parts,establish a dynamic model for efficient prediction,and verify it through simulation and experimentation.The proposed method yields remarkable results,with an average relative error of only 3.85％in modal frequency prediction and a staggering 99.7％reduction in solution time compared to finite element analysis.We further discuss the dynamic performance of the machine tool under varied stacking angles and layer numbers of the composite machine tool.We propose general design criteria for composite machine tools to consider the modal frequency and manufacturing cost of machine tools.
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